1. Field of the Invention
The present invention relates to an illumination system to project a light beam on a document surface, an image reader including the illumination system, and an image forming device including the image reader such as a copier, printer, facsimile machine, plotter, and a complex machine thereof.
2. Description of the Related Art
In related art an image scanner includes a light source to project a light beam to a document surface on a contact glass, and an imaging lens to collect a reflected light from the document surface travelling on the optical axis, and an image sensor or CCD (charge coupled device) to form an image. A light source of such an image scanner is a rod-like light source as a fluorescent lamp or xenon lamp, or a point light source such as LED. The LED is mostly used in place of a xenon lamp for the purpose of increasing a rising speed, elongating a life span, and saving power.
FIG. 12 shows the structure of an illumination system of a related art image scanner using an LED. It includes a first scan unit 103 with a bracket 121 with a V-shape cross section on which an LED circuit board 123 is mounted to drive an LED 122. A reflector 118 is attached to the first scan unit 103 to reflect the light beam from the LED 122 with a good illuminance distribution and eliminate a shadow when scanning a document including a cut-and-paste portion.
This image scanner using a reduced optical system faces a problem that the light beam from the light source is largely attenuated in a long distance from the document surface to the CCD. Accordingly, the LED of the image scanner needs to be improved in illuminance. The light amount of a single LED is small and an illumination area is narrow. In view of this, multiple LEDs 122 are arranged in main scan direction to increase the illuminance on a document surface.
With the LEDs 122 in array, to achieve a good illuminance distribution on the document surface in sub scan direction, the LED array is preferably inclined by the bracket 121 to oppose the document surface. Further, the illuminance distribution on the document surface in the sub scan direction is preferably such that only an area E or an actual scan area is illuminated with the light beam. However, even using an inclined LED array, it is known that the areas other than the scan area E are illuminated with the light beam reflected by the reflector 118. This causes a problem in scanning an image including a small black portion placed between white portions, for example, that the reflected beam by the white portions enters the image sensor, increasing the output value of the black portion. Because of this, the black portion cannot be accurately reproduced.
To solve this problem, Japanese Patent Application Publication No. 2007-5860 (Reference 1) discloses an illumination unit which includes, on the exit surface of the LED array, an optical guide extending in the main scan direction to guide the light beam from the LED array to a scan area, in order to illuminate the scan area with even illuminance distribution, for example. Further, by adjusting the interval of the LED arrangement, an arbitrary illuminance distribution in the main scan direction can be achieved. Using this, a convergent element as a lens or lens unit to converge or image a reflected light from the document on the CCD is disposed so that the optical axis thereof coincides with the center of the optical guide and the arrangement of the LEDs is adjusted so that the closer the position of the LEDs to both ends in the main scan direction, the narrower the interval of the LEDs. Thereby, it is able to correct illuminance falloff of a convergent element by cosine fourth law effectively.
Furthermore, Japanese Patent Application Publication No. 2010-213039 (Reference 2) discloses an illumination unit including on the exit surface of an optical guide a diffuse structure with an uneven density to diffuse light to the document surface with a proper illuminance distribution. The diffuse structure is densely formed at the center and scarcely formed at the ends in the main scan direction to substantially reduce transmittance on the diffuse surface and create an illuminance distribution having high end portions.
Japanese Patent Application Publication No. 2011-114762 (Reference 3) discloses another illumination unit incorporating an anisotropic diffuse structure to diffuse beams in not-radially circular shape, for instance. With use of the anisotropic diffuse structure in an image scanner with a long-length image sensor, the amount of light in a long effective scan area in sub scan direction can be reduced, improving the efficiency of the illumination unit.
However, in correcting the cosine fourth law of a convergent element by changing the LED interval, there is a limit to the mounting interval of the LEDs at the ends. Therefore, the maximal number of LEDs mounted on the circuit board is determined by the angle of view of the convergent element and the interval limit of the LEDs. Especially, by use of a convergent element with a short focal point and a wide angle of view, the illuminance on an image sensor having passed through the convergent element is greatly reduced from the center to the ends in the main scan direction.
FIG. 13 shows a relation between the illuminance on the document surface and that on the image sensor when the LEDs are arranged with equal interval in the main scan direction. The reflection rate of the document is set to 1.0 and illuminance falloff by the convergent element includes the decrease by cosine fourth law for simplicity. It is seen from the drawing that the illuminance on the image sensor is greatly reduced towards the ends. To correct the illuminance on the image sensor, the illuminance distribution needs to increase from the center to the ends of the document in the main scan direction as shown in FIG. 14. However, at a half angle of view of 30°, 1.78 times larger illuminance is required for the ends of the document in comparison with that for the center.
Accordingly, for correcting an illuminance difference on the document by changing the LED interval, it is necessary to set the interval at the center to be 1.78 times larger than at the ends in the main scan direction since the illuminance at some main scan position is approximately inversely proportional to the interval of the LEDs at that position. The illuminance on the image sensor is even at the value of the center of the document if the reflection rate on the document and the transmittance and reflection rate of the convergent element are even. That is, after the cosine fourth law correction, the illuminance on the document becomes even at the lowest value. In a scan system using a convergent element with a short focal point and a wide angle of view, the LEDs are very scarcely arranged at the center in main scan direction so that the larger the angle of view, the smaller the illuminance on the document by the central LEDs after the cosine fourth law correction. Therefore, the illumination unit having the convergent element with a short focal point is not applicable to a high-speed scanner requiring a large amount of light.
In Reference 2 the illuminance distribution on the document surface is created by changing the ratio of diffused beams by the density of the diffuse structure. The transmittance is decreased at the center of the diffuse structure due to a high density but the amount of light illuminating the effective scan area is also decreased, reducing the optical use efficiency. Therefore, the illumination unit cannot deal with high-speed scanning requiring a large amount of light.
In Reference 3, with the LEDs arranged at equal interval, a loss of light in sub scan direction can be reduced, however, the divergence angle in the main scan direction becomes constant and so does the illuminance on the document. Accordingly, the illuminance distribution for the cosine fourth law correction as shown in FIG. 14 cannot be created unless the mounting interval of the LEDs is changed or the number thereof is reduced. This illuminance unit cannot deal with high-speed scanning requiring a large amount of light either.